1. Field of the Invention
The present invention relates to a magnetic-optical recording medium and a magneto-optical recording method.
2. Discussion of Background
Recently, magneto-optical recording media utilizing magneto-optical effects which can be used as rewritable optical recording media have been actively developed and some of them have been actually used in practice. Such magneto-optical recording media are expected to be usable as media for filing documents, video and still images, and as memory medium for computers because of the advantages over other recording media that high volume, high density recording, and non-contact recording and reproduction can be carried out with extremely easy access, and that overwriting is also possible.
There are several technical problems to be solved in order to develop a magneto-optical recording medium to a recording medium having a recording performance equivalent to or better than that of a magnetic disk.
One of the most important problems relates to the overwriting techniques. Currently proposed overwrite techniques can be classified into two categories in accordance with the recording methods thereof. One is a magnetic field modulation method, and the other is an optical modulation method (a multi-beam method, a two-film-layered method or the like).
The magnetic field modulation method is a method of carrying out recording by inverting the polarity of an applied magnetic field in accordance with the information to be recorded. In this method, since the polarity of the applied magnetic field has to be inverted at high speed, a floating type magnetic head is required, so that the replacement of the recording medium employed is difficult.
On the other hand, the optical modulation method is a method of carrying out recording by switching the application of a laser beam on or off or subjecting a laser beam to be applied to intensity modulation in accordance with the information to be recorded.
A multi-beam method which belongs to this optical modulation method is a pseudo-over-write method in which two to three laser beams are employed, and the direction of the magnetic field is inverted at every rotation of the recording medium so that recording and erasure are conducted for each track in the recording medium. This method, however, has the shortcomings that the apparatus for use in this method requires a complicated structure, and is costly.
A two-film-layered method which also belongs to the optical modulation method employs a magneto-optical recording medium with a two-film-layered recording layer by which overwriting is carried out.
An example of the two-film-layered method is disclosed in Japanese Laid-Open Patent Application 62-175948. The two-film-layered method disclosed in the above Japanese Laid-Open Patent Application employs a magneto-optical recording medium provided with a two-film-layered recording layer which is composed of a memory layer comprising TbFe and an auxiliary layer comprising TbFeCo. In this method, the above magneto-optical recording medium is initialized, and then overwriting is carried out by the application of an external magnetic field and laser beams with different powers to the recording medium.
More specifically, in this method, prior to recording, the auxiliary layer is magnetized in a predetermined one direction by the application of a magnetic field for initialization, the temperature (T) of the recording medium is raised to a temperature Tc.sub.2 which is the Curie temperature of the auxiliary layer, that is, in the relationship of T&gt;Tc.sub.2, by the application of a high-power-output laser beam, so that the magnetization of the auxiliary layer is inverted by the application of a magnetic field for recording, which has a magnetization direction opposite to that of the magnetic field for initialization, and the magnetization of the auxiliary layer is transferred to a memory layer of the recording medium when the recording medium is cooled, whereby recording is carried out. Recorded information is erased by raising the temperature (T) of the recording medium to a temperature in the relationship of Tc.sub.1 &lt;T&lt;Tc.sub.2, wherein Tc.sub.1 is the Curie temperature of the memory layer, and the magnetization direction of the auxiliary layer is transferred to the memory layer. Because of the above steps for recording and erasing, the above-method requires a magnet for initialization.